1. Field of the Invention
The invention relates to the field of random access memory (RAM) devices formed using chalcogenide glass layers, and in particular to an improved programmable conductor random access memory (PCRAM) element design utilizing germanium-selenide glass layers.
2. Description of the Related Art
Programmable conductor memory elements recently have been investigated for suitability as semi-volatile and non-volatile random access memory devices. One known PCRAM element composition utilizes a germanium-selenide chalcogenide glass of GexSe100−x stoichiometry. A metal, such as silver, is incorporated into the germanium-selenide glass. It is believed that the metal provides conductivity to the element thus allowing the element to be switched between two resistance states. Particularly, a silver-containing GexSe100−x glass layer is positioned between two electrodes utilized in a PCRAM element.
The resistance of the silver-containing germanium-selenide glass layer can be changed between high resistance and low resistance states. The programmable conductor memory element is normally in a high resistance state when at rest. A write operation to a low resistance state is performed by applying a voltage potential across the two electrodes. When set in a low resistance state, the state of the memory element will remain intact for days or weeks after the voltage potentials are removed. The memory element can be returned to its high resistance state by applying a reverse voltage potential between the electrodes as used to write the element to the low resistance state. Again, the highly resistant state is maintained once the voltage potential is removed. This way, such a device can function, for example, as a resistance variable memory element having two resistance states, which can represent two logic states.
The stoichiometry of a GexSe100−x glass inherently provides many different glass compositions depending on the value of x. Certain structural characteristics of GexSe100−x have been observed with a change in the value of x. Specifically, referring now to prior art FIG. 1 (adapted from Boolchand, P., et al. “Onset of Rigidity in Steps in Chalcogenide Glasses—The Intermediate Phase” in M. Thorpe (ed.) Properties and Applications of Amorphous Materials, NATO Science Series (Plenum/Kluwer, 2001)), GexSe100−x glasses that are selenium-rich, i.e., glasses having a stoichiometry whereby x is less than or equal to 20, have a loose or open glass matrix as a result of the higher proportion of Se—Se bonds which occur when the relative number of germanium atoms is low. By contrast, GexSe100−x glass with values of x greater than 26 have a tight or rigid glass matrix due to the greater proportion of Ge—Se bonds present in the glass. GexSe100−x glass with values of x between 20 and 26 has an intermediate glass matrix. Rigidity is relative and based on the stoichiometry of the GexSe100−x glass. Accordingly, a Ge40Se60 glass is more rigid than a Ge33Se67 glass, and Ge25Se75 glass is more rigid than Ge20Se80 glass. Glass matrix structure is also relative and a function of the value of x. Therefore, a Ge40Se60 glass has a tighter glass matrix structure than a Ge33Se67 glass, and Ge25Se75 glass has a tighter glass matrix structure than Ge20Se80 glass.
The structure of the glass matrix, i.e., tight versus open, affects the switching characteristics of the memory element. If the GexSe100−x glass has a tight matrix, then a larger resistance change is inhibited when a memory element switches from an on to an off state. On the other hand, if the germanium-selenide glass matrix is looser, or more open, then a larger resistance change is more easily facilitated. Accordingly, since glasses having a tight matrix inhibit a large resistance change, a PCRAM element utilizing a glass with a tight matrix will keep the programmed state longer than a PCRAM element utilizing a glass with an open matrix.
While a PCRAM element comprised exclusively of a glass with an open matrix will be placed in a low resistance state or a high resistance state more quickly than a PCRAM element comprised exclusively of a glass with a tight matrix, glass with an open matrix used by itself is also not ideal. First, a PCRAM element utilizing glass with an open matrix may allow multiple low resistive pathways to be formed during programming. The tighter glass matrix will allow formation of a preferred conductive pathway with the properties of improved switching reliability and reduced resistance drift because there are fewer resistively decaying conductive pathways. Furthermore, the low melting point of selenium-rich glass, i.e., glass with an open matrix makes fabrication of PCRAM elements containing only glass with an open matrix difficult and complicated.
What is needed is a design for a PCRAM element which provides faster operation and better data retention than that which can be obtained with either open-matrix or tight-matrix glass.